Allow rendering output of any version of graph

[jonnor]

Right now we send URLs that represent output data, as a promise to render it when requested. However if the graph changes inbetween invalidation and request, the URL will give different results.
The url should contain a graph uuid and a revision, however then we need a way to get other versions of graph efficiently. Proposed solution is to use the currently unimplemented GEGL journal for this.

[jonnor]

As a hack one could just store every change as a new .json, named by graph + a monotonic counter.